Valves are nowadays used for a plethora of technical applications. They are used, for example, for opening or closing fluid connections between two points, or to selectively distribute fluid, entering the valve one side, to one or several of a plurality of fluid outlets of the valve. Not only flowing liquids are influenced by such valves, but also gases, liquids, mixtures of gases and liquids, mixtures of gases and solid particles (smoke), mixtures of liquids and solid particles (suspension) or even mixtures of gases, liquids and solid particles.
Generally speaking, two different kinds of valves exist: passive valves and active valves.
Passive valves generally change their position under the influence of the fluid itself. An example: spring loaded poppet valves, which open in one direction under the influence of a pressure difference between the fluid inlet port and the fluid outlet port of the valve. In the opposite direction, however, they remain closed irrespective of the pressure difference.
Active valves change their position under the influence of an externally applied actuation signal. In principle, the actuation signal can be the manual input of an operator. If the actuation of the valve has to be automated in some way, however, an automated way of actuating the valve is required. In the state of the art, several methods of actuating an actuated valve are known. For example, the actuated valve can be actuated by mechanical means. An example for this is the actuation of the valves of a combustion engine of a motor vehicle, where the valves are usually actuated mechanically by cams, mounted on a camshaft.
Another common way of applying an actuation signal to an actuated valve is the use of an electric current. This is particularly useful, when the actuation of the valves is controlled by an electric or electronic controlling unit. Here, the controlling unit will always generate an electric output signal initially. If a valve is used, which needs an actuation signal different from an electric current, an additional device is needed for converting the electric actuation signal into a different form of actuation signal. Of course, this is not desired.
Furthermore, an electric actuation of valves is quite often preferred, because such actuated valves are normally comparatively fast and accurate. Also, electric signals can be amplified easily.
Valve actuators for actuating valves, which use an electric current as an actuation signal, are known in the state of the art.
In GB 2 213 650 A, for example, a fuel injection valve is described, which can be actuated by externally applied electric signals. The fuel injection valve, disclosed in GB 2 213 650 A, includes an outer body and a core member extending within the body. The body and core member are formed from magnetizable material and define pole faces presented to a valve member which is spring biased into contact with a seating. Interposed between the body and core member is a permanent magnet which drives magnetic flux through the core member and body. The flux produced by the magnet is sufficient to hold the valve member in contact with pole faces, but not sufficient to lift the valve member from the seating. The flux produced by the magnet is enhanced when a first coil is energized to lift the valve member. When a second coil is energized the flux produced by the magnet is opposed to allow the spring to return the valve to the seating.
In WO 2007/128977 A2 another electro magnetic actuator is disclosed. The electro magnetic actuator comprises a core, a ferromagnetic component movable in a gap in the core, and a permanent magnet for attracting the component to one side of the gap. A flux concentrator concentrates the magnetic flux on that side of the gap and a main solenoid produces a magnetic flux in the gap. A magnetic circuit of the solenoid is defined by part of the core, part of the gap and by a further gap between the ferromagnetic component and the core. A demagnetizing coil has a magnetic circuit defined by another part of the core, another part of the gap and by the further gap. The demagnetizing coil is arranged to demagnetize the permanent magnet at least to the extent that the magnetic flux produced by the main solenoid is diverted from the flux concentrator into the further gap and the movable component is movable away from the permanent magnet under the magnetic force of the main solenoid. The current in the main solenoid, and therefore the magnetic force derived from it, rises relatively slowly. This is due to the size of the main solenoid which has to be of a large size to generate enough force to move the movable ferromagnetic component quickly. The demagnetizing coil is activated after the main solenoid has reached a high current, so that it is able to move the movable ferromagnetic component quickly.
In those actuators, two different coils are used. The presence of two different coils increases the actuators in size and weight. Furthermore, the actuators become more complex and more expensive. Another problem is that the generation and application of the different actuation signals can be difficult to achieve. Another problem can arise if the valve actuators are used for special technical fields. For example, in the field of hydraulic fluid working machines, more precisely in the field of synthetically commutated hydraulic fluid working machines, the requirements for the valve actuators are very stringent. Firstly, the valve actuators have to be very fast when opening or closing. Secondly, the response to the actuation signal (opening and closing) has to be very accurate and reproducible. Thirdly, the valve actuators have to be able to produce relatively strong forces. In particular, the holding force in the open position has to be relatively high (in the order of 80 N) to avoid self-closing of the valve due to flow forces, acting on the valve head especially at high rotational speeds and high oil viscosities. Of course, the power consumption of the valve actuator should be as low as possible.
In U.S. Pat. No. 7,077,378 B2, a valve assembly operable to allow or prevent the flow of fluid to or from a working chamber of a fluid-operated machine, comprising radially spaced apart inner and outer annular valve seats defining an annular passage therebetween, a valve member comprising a sealing ring, and means for moving the valve member axially between a first position in which the sealing ring is in seating engagement with the annular valve seats to close the annular passage to fluid flow therethrough and a second position in which the sealing ring is spaced from the annular valve seats so that the annular passage is open to fluid flow therethrough, is disclosed. The valve assembly comprises a body of a ferrous material. A movable pole member, commonly referred to as an armature is movably arranged within a gap of said body. Said movable pole member can be latched to said second position by a permanent magnet. By application of an electric current to a coil, said movable pole member can be delatched and moved to said first position. The middle “bridge” portion of the body, leading sideways to the movable pole member is dimensioned in a way that it cannot be driven into saturation, even if the maximum allowable current through the coil is applied. However, due to the finite gap between the bridge portion of the magnetic core and the body, a certain constant resistivity towards the penetration of magnetic flux is present. Therefore, a constant fraction (as long as the movable pole member has not yet moved) of the magnetic flux, generated by the electric coil, is going through the permanent magnet part of the magnetic core.
By the notion fluid working machines, hydraulic pumps, hydraulic motors, and machines, which can be used as pumps and as motors, are encompassed. Synthetically commutated hydraulic machines are also known as “digital displacement pumps”. Synthetically commutated hydraulic machines are known from EP 0361927 B1, EP 0494236 B1 and EP 1537333. They are a special subset of variable displacement fluid working machines.
Although such fluid working machines and/or such synthetically commutated hydraulic machines work well, both in theory and practical applications, there is still plenty of room for improvement, in particular for improvement in the field of the valve actuators.